The present invention relates to a magnetic recording medium, more particularly, to a magnetic recording medium capable of performing high-density recording by improving thermal stability of recording magnetization. Furthermore, a vertical magnetic recording method is one in which magnetic recording is carried out by applying recording magnetization in a direction vertical to a surface of a magnetic recording medium (an easy axis of magnetization), and one of the promising technologies for excellent magnetic recording methods supporting a recent trend of high density recording.
One example of a magnetic recording material will now be described, based on FIG. 1 showing a configuration of principal elements of a common vertical magnetic recording medium 100 in the prior art. As shown in FIG. 1, for example, the vertical magnetic recording medium 100 has a structure formed by laminating a soft magnetic underlying layer 102 consisting of nickel-iron, etc., a crystal controlling layer 103 consisting of chromium (Cr) and titanium (Ti), etc. laid on for crystal controlling, a vertical magnetic recording layer 104 consisting of an alloy containing cobalt such as cobalt-chromium (Coxe2x80x94Cr), etc. in which magnetic recording is carried out, and a protecting layer 105 consisting of hard DLC (Diamond Like Carbon), etc., in order from the bottom, on a non-magnetic substrate 101 consisting of aluminum, etc.
Herein, the underlying layer 102 is a layer laid on for improving recording sensitivity and this layer is not an essential layer for the vertical magnetic recording medium 100. Furthermore, in order to improve good crystallization and adhesion, a layer consisting of chromium or titanium, etc. may be formed before film formation of the respective magnetic layers.
In the vertical magnetic recording medium 100, for attainment of both high density recording and decrease of noise level, miniaturization and homogenization of magnetic particle diameters and elimination (segregation) of magnetic interaction between magnetic particles in the vertical magnetic recording layer 104, etc. are required and various investigations are made for them.
It is known that as the miniaturization and homogenization of the magnetic particle diameter and the elimination of the magnetic interaction between the magnetic particles in the vertical magnetic recording layer 104 are carried out, recording magnetization is destabilized by heat. Consequently, a design to produce the vertical magnetic recording medium 100 having large vertical coercive magnetic force Hc is required.
However, in the vertical magnetic recording medium 100 in the prior art, the vertical coercive magnetic force Hc was also decreased by heat causing a temperature rise.
This influence of the heat will now be described in detail. Since recording and reading (reproducing) are carried out in a vertical magnetic recording medium, it is designed so as to have, for example, approximately 2800 Oe as the maximum vertical coercive magnetic force Hc within a range in which magnetic recording is allowed.
FIG. 2 is a diagram showing a relationship between temperature (xc2x0 C.) and vertical coercive magnetic force (Hc) with respect to a common vertical magnetic recording medium in the prior art. As clearly shown in the figure, the vertical coercive magnetic force decreases with temperature rise. Hence it is recognized that recording magnetization of a vertical magnetic recording medium is destabilized by temperature rise.
Also, FIG. 3 is a diagram showing change of residual magnetization Mr with time at certain temperatures with respect to a vertical magnetic recording medium produced by eliminating the underlying layer 102 from the vertical magnetic recording medium 100 shown in FIG. 1. Herein, the vertical axis shows relative change, provided that the residual magnetization Mr in the vertical direction from the vertical magnetic recording layer 104 in an early stage is one. When one second after beginning of a measurement is defined as the time standard and the obtained data are extrapolated, the appearance of the change with time is shown. The filled triangle indicates at room temperature RT (approximately 25xc2x0 C.) and the filled dot indicates at 75xc2x0 C. that is assumed to be the guaranteed temperature of the vertical magnetic recording medium. The dashed line indicates a tolerance of temperature for maintaining a function of the vertical magnetic recording medium.
As clearly shown in FIG. 3, although the residual magnetization Mr hardly decreases at the room temperature and there is no problem, the decrease of the magnetization become significant and dips below the tolerance at 75xc2x0 C.
The decrease of magnetization in a medium caused by temperature rise or heat as described above is a phenomenon known as the thermal fluctuation magnetic after effect or the thermal magnetic relaxation.
That is, magnetization within a magnetic particle in a single magnetic domain is stabilized such that various magnetic energies represented by an anisotropic energy are minimized at lower temperature. Such condition of the magnetization is conceptually like a condition on which an inner part surrounded by magnetic barrier xcex94E is stabilized. Also, it is known that energy is added to a magnetization spin as thermal energy as temperature is raised and the magnetization comes to be in a disordered state as thermal energy kT (k is Boltzmann constant) is larger than the energy barrier xcex94E.
However, according to the statistical mechanics, even if the thermal energy is not so large, the thermal energy kT may randomly exceed the magnetic energy barrier xcex94E. The larger the thermal energy is, the smaller the thermal energy barrier xcex94E is, and the longer elapsed time is, the more the probability increases. Usually, if temperature is constant, the magnetic energy barrier xcex94E and the thermal energy kT are approximately constant. Hence, with respect to magnetization spins directed to one direction by means of magnetic recording, magnetization spins in the random state increase with time. Therefore, it seems that the magnetization decreases with time. This is a phenomenon referred to as the thermal magnetic relaxation.
In the case of the vertical magnetic recording medium, as the influence of the phenomenon is considered, the magnetic energy barrier xcex94E strongly depends on vertical coercive magnetic force Hc along the vertical direction, and the higher the vertical coercive magnetic force Hc is, the higher the magnetic energy barrier xcex94E is.
In the previously described vertical magnetic recording medium in the prior art, since temperature inside a magnetic recording medium driver rises, not only does thermal energy kT increase, but also a magnetic energy barrier xcex94E dependent on vertical coercive magnetic force Hc decreases due to a decrease of vertical coercive magnetic force and the phenomenon of thermal magnetic relaxation easily occurs at higher temperature.
As clearly seen from the previous illustration, in the vertical magnetic recording medium 100 in the prior art, the vertical coercive magnetic force Hc in the vertical magnetic recording layer 104 decreases with heat causing a temperature rise so that the magnetization condition is destabilized. Then, such decrease of the coercive magnetic force caused by heat is also problematic in a magnetic recording medium for another magnetic recording method.
Therefore, it is an object of the present invention to provide a magnetic recording medium capable of attaining high density using a magnetic material that is thermally stabilized by an increase of coercive magnetic force in a recording magnetic layer with temperature rise.
The object can be achieved by a magnetic recording medium comprising a recording magnetic layer in which magnetic recording is carried out, wherein the recording magnetic layer is set such that coercive magnetic force increases with temperature rise within an operating temperature range in which the magnetic recording medium is used.
In the above mentioned invention, the magnetic recording medium is used in the magnetic recording medium driver and, as the temperature inside the driver rises, accordingly the coercive magnetic force in the recording magnetic layer increases. Hence, the coercive magnetic force in the recording magnetic layer increases with temperature rise to stabilize recording magnetization, contrary to the prior art. Therefore, miniaturization and homogenization of the recording magnetic layer can be achieved and a high-density magnetic recording medium can be provided.
In the present specification, the operating temperature is a temperature at which a magnetic recording medium is used in a magnetic recording medium driver. A supposed operating temperature range is different, dependent on the environment in which a magnetic recording medium is used. For example, when a magnetic recording medium driver is used in the environment in which room temperature is supposed to be 15 through 30xc2x0 C., there is a possibility that temperature inside the driver rises to approximately 70 through approximately 80xc2x0 C. as this room temperature is a lower limit. Therefore, in the above case, a temperature range from room temperature at time of starting of a magnetic recording medium driver to higher temperature during driving, for example, 15xc2x0 C. through 80xc2x0 C., is the operating temperature range of the magnetic recording medium.
Herein, an upper limit of the temperature range is generally called a guaranteed temperature in terms of guaranteeing the function (of maintaining recording magnetization) when a magnetic recording material is used in a magnetic recording medium driver at higher temperature. Similarly, as use of a magnetic recording medium in an environment at lower temperature is supposed, guaranteed temperature at the low temperature side may be defined as a lower limit of operating temperature.
Furthermore, the object can be achieved by a magnetic recording medium comprising a recording magnetic layer in which magnetic recording is carried out, wherein the recording magnetic layer comprises an N-type Ferrimagnetic material and compensation temperature of the N-type Ferrimagnetic material is made to be higher than an operating temperature range in which the magnetic recording medium is used.
In the above mentioned invention, the operating temperature at which a magnetic recording medium is used is lower than compensation temperature Tcomp, however, as temperature inside a magnetic recording medium driver rises, the operating temperature approaches the compensation temperature Tcomp. At this compensation temperature Tcomp, coercive magnetic force of a recording magnetic layer becomes infinite resulting from a property of an N-type Ferrimagnetic material. Hence, the more operating temperature rises and approaches compensation temperature Tcomp, the stronger the coercive magnetic force becomes.
Since coercive magnetic force becomes stronger as temperature rises, recording magnetization can be stably maintained. Hence, it becomes possible to miniaturize and homogenize a recording magnetic layer, or to eliminate magnetic interaction between magnetic particles to provide a high-density magnetic recording medium.
Then, as the N-type Ferrimagnetic material has a configuration of comprising an amorphous alloy in which at least one selected from the rare earth element group consisting of gadolinium (Gd), terbium (Tb), neodymium (Nd), praseodymium (Pd), dysprosium (Dy), holmium (Ho) and erbium (Er), and at least one selected from the transition metal element group consisting of iron (Fe), cobalt (Co) and nickel (Ni) are combined, a magnetic recording medium using a more preferred vertical magnetic recording method can be formed.
Herein, by appropriately combining the rare earth element and the transition metal element, many N-type Ferrimagnetic materials can be designed such that vertical coercive magnetic force Hc of a recording magnetic layer becomes stronger as temperature rise. Also, although an appropriate selection should be made in order to obtain an N-type Ferrimagnetic material satisfying a necessary condition, it is preferable that at least one of gadolinium and terbium should be selected from the rare earth element group and at least one of iron and cobalt should be selected from the transition metal element group.
Then, the N-type Ferrimagnetic material can be made to have a configuration of having a composition proportion of the rare earth element such that the compensation temperature is made to be higher than the operating temperature range. Compensation temperature Tcomp of an N-type Ferrimagnet material can be changed by adjusting the composition, and when the composition proportion of the rare earth element is high, compensation temperature Tcomp can be made to be higher than the operating temperature at which a vertical magnetic recording medium is used.
Herein, operating temperature of a magnetic recording medium according to the present invention can be set at 0xc2x0 C. through 80xc2x0 C. As described above, operating temperature is a temperature at which a magnetic recording medium can be used inside a magnetic recording medium driver. An operating temperature range is appropriately set, taking the environment in which a magnetic recording medium is used into consideration. There is a setting of lower or higher temperature side, or a setting of width of operating temperature range, etc. Generally, from 0xc2x0 C. regarded as a guaranteed temperature at lower limit side to 80xc2x0 C. regarded as a guaranteed temperature of an upper limit side is the widest operating temperature range, and when a magnetic recording medium is designed, the operating temperature range is decided within this widest operating temperature range. Then, the compensation temperature should be higher than the operating temperature range set herein.
Also, a magnetic recording medium driver comprising the magnetic recording medium and a magneto resistive head is included in the present invention. Since the magnetic recording medium in which coercive magnetic force in a recording magnetic layer increases with temperature rise is comprised and this medium can be read by means of a high sensitivity magneto resistive head, magnetic recording information can be reproduced exactly and sensitively.